Major saccharides contained in lignocellulose are glucose that constitutes cellulose and xylose that constitutes hemicellulose. By chemically or enzymatically degrading lignocellulose, a saccharified composition mainly composed of such a monosaccharide can be obtained. Production of useful materials from lignocellulose at the industrial level necessitates microorganisms that are capable of effective utilization of saccharides contained in such saccharified compositions and fermentation of such useful substances with high yield and high productivity.
In general, yeasts with high ethanol fermentation ability, such as Saccharomyces cerevisiae, are capable of utilizing glucose, mannose, or galactose, although such yeasts are not capable of utilizing xylose. In order to perform fermentation using lignocellulose as a starting material with high efficiency, accordingly, it is necessary that such yeasts be modified to be capable of using xylose.
For example, production of recombinant S. cerevisiae capable of xylose utilization has been attempted (Patent Literature 1 and 2; Non-Patent Literature 1). Patent Literature 1 and Non-Patent Literature 1 each report improvement in ethanol yield and xylose utilization of a yeast to which xylose assimilability has been imparted through introduction of genes encoding xylose reductase (XR) and xylitol dehydrogenase (XDH) from different microorganisms. Such literature reports that the phosphoketolase (PK) pathway is potentiated and NADH is consumed by acetaldehyde dehydrogenase, in order to consume excessive NADH generated by introduction of a xylose assimilation pathway caused by XR and XDH in the subsequent reaction.
Patent Literature 2 reports the use of xylose isomerase (XI), which is an isomerase that converts xylose into xylulose. When XI is used, excessive NADH is not generated. That is, the glycolytic pentose phosphate pathway (PPP) is used without any processing, and the phosphoketolase pathway is not potentiated in the technique according to Patent Literature 2.
While NADH may be excessively consumed as a result of introduction of the acetaldehyde dehydrogenase gene into a xylose-assimilating yeast into which the XI gene had been introduced, the improvement of xylose assimilation as a result of introduction of such gene has also been reported (Patent Literature 3).